In CT X-ray imaging of a patient, X-rays are used to image internal structure and features of a region of the person's body. The imaging is performed by a CT-imaging system, hereinafter referred to as a “CT scanner” that images internal structure and features of a plurality of contiguous, relatively thin planar slices of the body region using X-rays.
The CT scanner generally comprises an X-ray source that provides a planar, fan-shaped X-ray beam emanating from a focal spot of the X-ray source and an array of closely spaced X-ray detectors that are substantially coplanar with the fan-beam and face the X-ray source. The X-ray source and array of detectors are mounted in a gantry so that a person being imaged with the CT scanner, generally lying on an appropriate support couch, can be positioned within the gantry between the X-ray source and the array of detectors. The gantry and couch are moveable relative to each other so that the X-ray source and detector array can be positioned axially, along a “z-axis”, at desired locations along the patient's body.
The gantry comprises a stationary structure, referred to as a stator, and a rotary element, referred to as a rotor, which is mounted to the stator so that the rotor is rotatable about the z-axis. In third generation CT scanners the X-ray source and detectors are mounted to the rotor. The detectors are generally arrayed along an arc of a circle having its plane perpendicular to the z-axis and its center located at a focal spot of the scanner's X-ray source. Hereinafter, such an array of detectors along an arc of a circle is referred to as a “row” of detectors. Angular position of the rotor about the z-axis is controllable so that the X-ray source can be positioned at desired angles, referred to as “view angles”, around the patient's body. In fourth generation CT scanners the X-ray detector array comprises detectors positioned around the perimeter of a circle to form a full circle of detectors. The circle of detectors is stationary and the X-ray source is mounted to the rotor and rotates with the rotor.
To image a slice in a region of a patient's body, the X-ray source is positioned at the z-axis coordinate of the slice and rotated around the slice through an angle of at least 180° to illuminate the slice with X-rays from a plurality of different view angles. At each view angle, detectors in the array of detectors measure intensity of X-rays from the source that pass through the slice. The intensity of X-rays measured by a particular detector in the array of detectors is a function of an amount by which X-rays are attenuated by material in the slice along a straight-line path length from the X-ray source, through the particular slice, to the detector.
The measurements provided by the X-ray detectors are processed using algorithms known in the art to provide a map of the absorption coefficient of the material in the slice as a function of position. Maps of the absorption coefficient for the plurality of contiguous slices in the region of the patient's body are used to display and identify internal organs and features of the region.
Many older CT scanners are single slice scanners that comprise only a single row (i.e. also a single circle for fourth generation CT scanners) of X-ray detectors and image only one slice of a region of a person's body at a time. Most modem CT scanners are multislice CT scanners designed to simultaneously image a plurality of slices of a patient. A multislice CT scanner comprises a plurality of parallel rows (or circles for fourth generation scanners) of X-ray detectors closely spaced one next to the other along the z-axis. The detector array of a multislice CT scanner is thus a “two-dimensional” (ignoring the curvature of the rows) matrix of rows and columns of detectors. A column of detectors in the array refers to detectors comprised in the rows of detectors that lie along a same line parallel to the z-axis of the scanner.
A multislice scanner can be operated to simultaneously image a number of slices of a patient up to a maximum number of slices equal to the number of rows of detectors. Typically, signals from detectors in a multislice scanner are combined in accordance with any of various algorithms known in the art to simultaneously image a plurality of slices that is less than the number of rows of detectors. A present day conventional multislice scanner may image as many as four to sixteen slices of a region of a patient simultaneously and scanners that simultaneously image more than sixteen slices are under development.
Ideally, each detector in a CT scanner measures intensity of X-rays that reach the detector after passage along a substantially straight-line path from the X-ray source to the detector. Therefore, ideally, only those X-rays that are neither absorbed by the material along the path from the X-ray source to the detector nor scattered by the material at angles that prevent the X-rays from being incident on the detector reach the detector. However, X-rays that are scattered out of the path from the X-ray source to one X-ray detector in the detector array of the CT scanner are generally scattered in directions towards other X-ray detectors in the scanner detector array. If these scattered X-rays are incident on the other X-ray detectors, they can generate error in measurements provided by the other detectors and degrade quality of an image provided by the CT scanner.
To reduce “scattering errors” in a CT scanner, X-ray detectors in the scanner's detector array are generally shielded from scattered X-rays. “Anti-scattering” shielding, hereinafter “AS shielding”, for a detector array of a multislice CT scanner usually comprises a thin planar baffle foil, hereinafter an “AS foil”, formed from a suitable X-ray absorbing material positioned between each column of detectors in the array. The plane of each AS foil is parallel to the z-axis and oriented so that it intersects the focal spot of the X-ray source, or a centroid of the focal spot if a deflecting focal spot is used. Hereinafter, a focal spot of an X-ray source is understood to mean a focal spot or, for a CT scanner having a deflecting focal spot, the centroid of the focal spot. For single slice CT scanners the AS foils are relatively effective in moderating effects of scattered X-rays on detector accuracy and image quality. However, the X-ray fan-beam in a multislice scanner is substantially thicker along the z-axis than the X-ray fan-beam in single slice scanners. As a result, detectors in a multislice CT scanner are potentially exposed to substantially more sources of scattered X-rays and thereby to a greater flux of scattered X-rays than are detectors in a single slice CT scanner.
To cope with increased flux of scattered X-rays in a multislice CT scanner, AS foils in a multislice scanner are often made higher than AS foils in a typical single slice scanner. However, a height to which AS foils can be fabricated is limited by production tolerances. As the height of AS foils is increased, the foils have to be fabricated and positioned to ever-finer tolerances. For example, as the height of AS foils in a CT scanner increases, the AS foils have to be aligned with the focal spot of the scanner's X-ray source to a greater degree of accuracy. To an extent that an AS foil is misaligned with the focal spot or its surface is compromised by inhomogeneities, it may shadow to a greater extent X-ray detectors adjacent to the AS foil. Shadowing alters the detection efficiency of a shadowed X-ray detector and may generate artifacts in images generated by the multislice CT scanner. In addition, shadowing, when it exists, is often unstable and may change from time to time. As a result, it can be difficult to compensate images provided by the CT scanner for artifacts introduced by shadowing.
It is noted that because of the geometry of fourth generation CT scanners, it is generally not possible to provide anti-scattering shielding for detectors comprised in a fourth generation CT scanner that does not result in undesirable shadowing of the detectors. As a result anti-scattering shielding is usually not provided for fourth generation scanners.
For multislice scanners, as the number of slices simultaneously imaged increases, the problem of scattering error is exacerbated As it does not seem possible to deal with the problem simply by increasing the height of AS foils used to shield the detectors, new solutions to scattering error are required.